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Study Area: Ground Avionics Engineering
Phase I: Basic ATSEP Course
Stream: Basic Principles of Distance Measuring
Equipment, DME
Instructor: Robert Jere Omusonga
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Objectives
1. State the purpose of
DME
2. State the principle of
DME
3. Explain the principle of
operation of DME
4. Determine the slant
distance
5. Describe signals used
in DME
6. Discuss how overload
is controlled
7. Discuss how echo and
multipath errors are
controlled
8. Outline the monitored
parameters of DME
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IntroductionNavaids:
Radio communication systems are used to
provide navigation guidance to flights.
Navaids include NDB, DME, VOR, ILS and
GNSS.
This lesson:
Introduction to basic principles of DME
Previous lessons:
Introduction to radio navigation
Principles of NDB
4
Meaning of DME
DME
Distance Measuring Equipment
i.e.
Equipment that measures distance
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DME
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State the Purpose of DME
Main Purpose
To provide distance information between a flying
aircraft and a DME ground station
Minor purposes
- provide time estimate to the DME station
- calculate speed of aircraft
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Feedback
1.The DME is a radio navigation system that
provides information pertaining to distance
from:
A. DME ground station to the airport
B. Flying aircraft to ATC control tower
C. DME ground station to the flying aircraft
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State the Principle of DME
DME principle is based on derivation of
distance using time and speed of an
electromagnetic wave
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Feedback
The principle of DME is based on:
A. Calculation of distance using time and
speed of the aircraft
B. Calculation of distance using time and
speed of RF waves
C. Calculation of distance using time and
speed of sound waves
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Simplified DME Airborne Instrument
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Simplified Block diagram of DME
Ground Equipment
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1t2t
StttT 5021
)103(2
50 8xSt
D T
Aircraft
DME transponder
S50
DME Interrogator
Explain the Principle of DMEFrequency range
962MHz to 1215MHz
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Feedback1.The standard system delay as used in
the principle of operation of DME
Channel X is:
A. 36uS
B. 12uS
C. 50uS
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Determine Slant Distance
Heig
ht a
bo
ve th
e
Gro
un
d
DMEActual Distance over Ground
D
H
V
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Determine Slant Distance
H
V
Slant error is most pronounced at high altitude
when close to the DME station
Pythagoras theorem:
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Feedback
Introduction to principles of DME 16
1.The range indicated by DME instrument
in the aircraft is:
A. Horizontal
B. Slant
C. Vertical
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Airborne Equipment
(Interrogator)
Internal
delay 50µs
Ground Equipment
(Transponder)
The Interrogation travels
at the speed of light
The Reply travels
at the speed of light
DME Signals
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Transmits omnidirectional
interrogations
Operates at approximately 27
interrogations per second.
It recognizes the replies to its own
interrogations by using unique PRF.
DME Signals
The Airborne Interrogator:
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Receives interrogations. Decodes and validates.
Interrogation Reply
Who’s it from?
Don’t know, don’t care.
All we care is,
it’s a valid
interrogation.
So we
add some
delay
time…
And then trigger
a reply.
Time from reception of interrogation to
emission of reply is 50µs (fixed).
DME Signals
The Transponder:
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X-Channel Interrogation Pulse Pair
50%
Width
3.5µs
90%
10%
Rise Time
2.5µs
Fall (Decay)
Time 2.5µs
Spacing 12µsX-Channel Reply
Pulse Pair is
identical to the
Interrogation Pair
Gaussian shaped pulses
Notice where to measure the
width: at the half-amplitude point.
DME Signals
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X-Channel Transmitter Delay
Interrogation Reply
Delay 50µs
50%
DME Signals
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24
Delay 56µs
50%
Y-Channel Pulses
Interrogation 36µs Reply 30µs
Pulse Width, Rise Time and Fall Time
are identical to X-Channel pulses
DME Signals
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1.The pulse spacing time as used in the
principle of operation of DME Channel X is:
A. 36uS
B. 12uS
C. 50uS
Feedback
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1.The periods of 3.5uS, 12uS and 50uS in DME
channel X respectively represent:
A. Pulse width, pulse spacing, system delay
B. System delay, pulse width, pulse spacing
C.Pulse spacing, pulse width, system delay
Feedback
27
The DME maintains a minimum duty
cycle of 800 pps in order for the aircraft
AGC to work efficiently.
Underload Control
What if interrogations fall
below 800pps?
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If interrogations fall below 800pps then squitter
pulses are randomly generated by
transponder to top-up so as to maintain
minimum replies of 800pps
Underload Control
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If there are enough replies to produce 800
or more reply pulse pairs per second (pps),
then no squitter pulses are produced.
Underload Control
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Feedback
1.The random pulses generated by the DME
to fill in and maintain a constant minimum
load are referred to as:
A. Receiver dead time pulses
B. Overload control pulses
C. Squitter pulses
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Modern DMEs limit the number of replies to
5400pps to avoid unacceptable level of
coincidental interrogations/replies.
5400 pps allows the DME to reply to about
200 airborne interrogators at a time since
each aircraft radiates at 27pps
i.e. 27 x 200 = 5400
Overload Control
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If interrogations are greater than 5400pps then the
transponder sensitivity is reduced by AGC. Range is
reduced to accommodate nearest 200 aircrafts only
The AGC is set to a level that eliminates
the weakest interrogations, allowing only
the strongest to pass through the receiver.
Overload Control
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1.If a DME transponder is limited to
maximum replies of 5400pps, calculate
the number of aircrafts whose
interrogation will be processed given that
each aircraft interrogates at 27pps
A. 100
B. 200
C. 800
Feedback
34
Feedback
1.If DME beacon becomes saturated, it
adjusts itself using AGC to give priority to:
A. the nearest aircrafts
B. the urgent aircrafts
C. the largest aircrafts
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Each DME identifies itself by transmitting a
Morse Code at 1350Hz in between
interrogations and replies
Each group consists of two
pulse pairs.Spacing 100µs
Period 741µs (Frequency 1350Hz)
DME Identification
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Every time the DME transmitter fires, for any
reason (monitor reply, aircraft reply, squitter or
identification), it transmits a pulse pair.
Pulse rise time, fall time, width, and spacing
remain constant.
Note
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RF Frequencies
The Interrogation is
transmitted by the
aircraft on one
frequency
Frequency range
962MHz to 1215MHz
The Reply is
transmitted by
the DME
Transponder on
another
frequency
Sometimes the Interrogation
frequency is higher,
sometimes the Reply
frequency is higher. The
difference between the two
frequencies is always 63MHz
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1.DME appears in the frequency band of:
A. VHF
B. HF
C. UHF
Feedback
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Building 2
A short distance echo to the
first interrogation pulse arrives
before the second interrogation
pulse arrives.
Transponder recognizes
this condition and mutes
its receiver for a specified
period of time (receiver
dead time) for echo to
pass, and replies only to
the real interrogation.
Echo Control
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Building 2
If the first echo overlaps the second pulse,
then only the echo would generate a reply.
If this condition were allowed, then it is
possible that the aircraft would receive
incorrect information.
12µs 12µs
Echo Control
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Building 2
Short distance echoes cause the
Airborne Interrogator to receive
unstable readings, jumping between
two values with a difference of about
1 nm (X channel) or 3 nm (Y
channel). Corrective action: enable Short
Distance Echo Suppression
(SDES) – also called receiver dead
time (RDT)
20nm? 19nm?
Echo Control
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A long distance echo to the first
interrogation pulse arrives after the
second interrogation pulse arrives.
Both the interrogation and the
echo generate a reply.
Echo Control
The corrective action shall be
discussed in next lesson
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Feedback
1.Echo errors in DME system are as a result
of:
A. Rough terrain
B. Doppler effect
C. Speed of the aircraft
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DME Monitored Parameters
Monitoring is necessary to ensure that DME
signal is correct and free of error
Parameters
1. Pulse spacing = 12uS
2. System delay = 50uS
3. Efficiency (% no. of replied inter) = 70%
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4. PRF (pulse repetition frequency) = 800
– 5400 pps
5. RF power output = 100W or 1000W
6. Identification = 1350 Hz
DME Monitored Parameters
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Feedback
1.Which of the following are the monitored
parameters of DME
A. Pulse spacing, 30HZ level, system delay
B. Pulse spacing, efficiency, system delay
C. RF power level, phase difference,
identification
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Review
1. State the
purpose
2. State the
principle
3. Explain the
principle
4. Determine slant
distance
5. Signals in DME
6. Overload control
7. Echo control
8. Monitored
parameters
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Next lesson
1. Block diagram of
DME interrogator
2. Block diagram of
DME transponder
3. Technical
description of
sequence of signal
flow
4. Co-location of DME
with ILS or VOR
5. Types of DMEs in
the market
6. Future DMEs
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Any Questions
Thank you
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Evaluation
1. With reference to distance measuring equipment
(a) State the purpose of DME in radio navigation (2 marks)
(b) Explain the following parameters as used in DME
(i) Pulse spacing (2 marks)
(ii) Pulse width (2 marks)
(iii) System delay (2marks)
(iv) Efficiency (2 marks)
(c) State the principle of operation of DME (4 marks)
(d) Describe in four steps the sequence of operation of DME system (8 marks)
(e) Discuss how the following problems are solved in DME systems
(i) Echoes and reflections (4 marks)
(ii) Overload arising from too many aircrafts (4 marks)
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